In general, an electrostatic actuator is provided with two electrodes opposing each other with a gap interposed therebetween, and a distance between the two electrodes changes by an action of electrostatic attractive force exerted therebetween when a driving voltage is applied.
FIGS. 8A and 8B are cross sectional views illustrating an example of a conventional variable capacitance element 3 using an electrostatic actuator.
Referring to FIGS. 8A and 8B, the variable capacitance element 3 is provided, on a substrate 31, with a fixed electrode 32, a dielectric layer 33 covering the fixed electrode 32, a movable electrode 34 opposing the dielectric layer 33 with a gap interposed therebetween, a pair of support layers 35a and 35b supporting the movable electrode 34, and so on.
A short circuit caused by contact between the fixed electrode 32 and the movable electrode 34 is prevented by the dielectric layer 33.
A driving voltage V can be applied between the fixed electrode 32 and the movable electrode 34.
Referring to FIG. 8A, when no potential difference is applied between the fixed electrode 32 and the movable electrode 34, i.e., when the driving voltage V is zero, the movable electrode 34 is in a position away from the fixed electrode 32.
On the other hand, referring to FIG. 8B, when a predetermined potential difference is applied between the fixed electrode 32 and the movable electrode 34, the movable electrode 34 is attracted toward the fixed electrode 32 and brought into contact with the dielectric layer 33.
A capacitance C between the fixed electrode 32 and the movable electrode 34 becomes a minimum capacitance Cs when the movable electrode 34 is in the state illustrated in FIG. 8A, and becomes a maximum capacitance Cg when the movable electrode 34 is in the state illustrated in FIG. 8B.
When the variable capacitance element 3 is used in a digital application, the capacitance C is changed either to the minimum capacitance Cs or the maximum capacitance Cg for use by controlling the driving voltage V.
FIG. 9 is a diagram illustrating an example of a relationship (C-V characteristics) between the capacitance C and the driving voltage V applied to the variable capacitance element 3.
As illustrated in FIG. 9, when the driving voltage V having a positive polarity is applied and increased, the capacitance C keeps the minimum capacitance Cs for a while, sharply increases when the driving voltage V passes a pull-in voltage Vpi1, and thereafter reaches the maximum capacitance Cg. Then, when the driving voltage V is decreased, the capacitance C keeps the maximum capacitance Cg for a while, sharply decreases when the driving voltage V passes a pull-off voltage Vpo1, and thereafter returns to the minimum capacitance Cs.
Since it is the potential difference that causes the electrostatic attractive force when applied between the fixed electrode 32 and the movable electrode 34, application of a driving voltage V having a negative polarity also brings about the same characteristics.
Here, the change in the capacitance C relative to the change in the driving voltage V has different paths, i.e., one when the driving voltage V increases and the other when the driving voltage V decreases. In other words, it exhibits a type of hysteresis phenomenon, and the pull-in voltage Vpi1 and the pull-off voltage Vpo1 are different from each other. The same is also applied to the pull-in voltage Vpi2 and the pull-off voltage Vpo2.
In the variable capacitance element 3, when the capacitance C is changed to the minimum capacitance Cs, the driving voltage V is set to an off-voltage Voff (zero). When the capacitance C is changed to the maximum capacitance Cg, the driving voltage V is set to an on-voltage Von1 or Von2.
If a driving voltage V with a same polarity is kept applied to change the capacitance C, then positive or negative charges accumulate in the dielectric layer 33, and the dielectric layer 33 is electrically charged.
FIGS. 10A and 10B are diagrams illustrating examples of a relationship (C-V characteristics) between the capacitance C and the driving voltage V of the variable capacitance element 3 while the dielectric layer 33 is kept electrically charged.
When the dielectric layer 33 is turned to a charged state, the movable electrode 34 is influenced by the electrostatic force caused by the charge. For this reason, the C-V characteristics of the variable capacitance element 3 exhibit characteristics different from those when the variable capacitance element 3 is not charged.
As illustrated in FIG. 10A, for example, in a state where the dielectric layer 33 is negatively charged, the C-V characteristics are shifted toward a negative side of the driving voltage V as compared with the state where the dielectric layer 33 is not charged. As illustrated in FIG. 10B, in a state where the dielectric layer 33 is positively charged, the C-V characteristics are shifted toward a positive side of the driving voltage V as compared with the state where the dielectric layer 33 is not charged.
If the C-V characteristics are shifted in this way, a driving voltage V that is supposed to cause the minimum capacitance Cs or the maximum capacitance Cg may not bring the capacitance C into such a value.
For example, referring to FIG. 10A, when the driving voltage V is set to the off-voltage Voff, the capacitance C may not change to the intended minimum capacitance Cs. This is because, for example, the C-V characteristics are shifted toward the negative side of the driving voltage V; the polarity of the pull-off voltage Vpo1 turns to negative from positive; and the movable electrode 34 is kept stuck onto the dielectric layer 33 even if the driving voltage V is brought back to the off-voltage Voff.
As described above, the charging phenomenon of the dielectric layer 33 works as an obstacle to the stable operation of the variable capacitance element 3.
According to “G. Papaioannou and J. Papapolymerou, Dielectric Charging in MEMS by Material, Structure and Temperature, in IEEE MTT-S International Microwave Symposium Workshop, June 2009”, the reason why the dielectric layer 33 is charged is explained as follows.
FIGS. 11A and 11B are diagrams schematically depicting the phenomenon in which charges accumulate in the dielectric layer 33.
As illustrated in FIGS. 11A and 11B, when the dielectric layer 33 is observed microscopically, asperities are present on the surface thereof.
For this reason, referring to FIG. 11A, when the movable electrode 34 is in contact with the dielectric layer 33, there are, on the surface of the dielectric layer 33, contact portions Tc which actually make contact with the movable electrode 34 and non-contact portions NTc which do not make contact with the movable electrode 34. In this state, positive or negative charges move to the contact portions Tc from the movable electrode 34, i.e., a type of current I flows. On the other hand, such charges do not move to the non-contact portions NTc.
Thereafter, when the movable electrode 34 moves away from the fixed electrode 32, electrostatic discharge is caused. However, a part of charges that have been injected in the vicinity of the contact portions Tc remains there without being released as illustrated in FIG. 11B.
This causes a difference in the state of charges between the vicinity of the contact portions TC and the vicinity of the non-contact portions NTc. As a result, the dielectric layer 33 is either positively or negatively charged.
Japanese Laid-open Patent Publication No. 2006-247820 proposes a driving method for switching the polarity of the driving voltage to be applied each time the driving is performed, i.e., bipolar driving.
In view of preventing the movable electrode from sticking to the fixed electrode by generation of forces such as the Van der Waals' forces, Japanese Laid-open Patent Publication No. 2004-61937 proposes a similar driving method.
Also, Japanese Laid-open Patent Publication No. 2007-242607 proposes to detect an amount of charges accumulated in an insulating film, and vary a driving voltage in accordance with the detected result.
Assuming that the above-mentioned contact portions correspond to identical locations in each of the driving, it appears that charging of the dielectric layer can be suppressed because charges that move through the contact portions cancel each other by performing the bipolar driving.
In actual cases, however, the contact portions do not necessarily correspond to the identical locations in each of the driving. Therefore, the bipolar driving does not work as an effective solution.
Further, it is complicated and troublesome to implement a method for varying the driving voltage in accordance with the result obtained by detecting an among of charges accumulated in the dielectric layer, because such a method requires a circuit for detecting the charges and various types of control related thereto.